Mechanical and corrosion behavior of thick and soft DLC coatings

Abstract

The DLC coatings are chemically inert, have low friction coefficient and good wear resistance. Depending on H content and sp3 bonds, they can be classified in “soft” or “hard” films. In this work, the corrosion and mechanical behavior, as well as the adhesion of thick DLC coatings are studied. The coatings, which are in fact silicon containing amorphous hydrogenated carbon films, were deposited by PACVD on nitrided austenitic stainless steel (duplex sample) and non-nitrided austenitic stainless steel (coated sample). The films were characterized by EDS and Raman spectroscopy, hardness was assessed by nanoindentation and microstructure was analysed by OM and SEM. To evaluate sliding wear behavior and friction, pin on disk tests were performed. The abrasive wear resistance was tested using the ASTM G65-95 Dry Sand/Rubber Wheel test. Erosion tests were conducted in water and sand flux. The corrosion resistance was evaluated by the Salt Spray Fog Test and electrochemical tests. The adhesion was tested using Scratch Test. The purpose was to systematically characterize the defects present in these thick DLC coatings relating them to adhesion, wear and corrosion resistance. The thickness of the DLC coating was about 37 μm, and its hardness was 12 GPa. The coatings presented a low friction coefficient, about 0.09. In the abrasive tests, the mass loss was negligible and in erosion experiments, the mass loss was reduced to 30%. Regarding the corrosion performance, the behavior in the Salt Spray Fog Test was good only in the duplex samples. In potentiostatic tests in 3.5% NaCl using steps, the current density increased at higher potentials for the duplex sample. Concerning the film adhesion, the critical load was also higher in theses samples than in the coated ones. The nitrided layer was a good interface for reducing the stresses and improving the adhesion, which is relevant for the decrease of the propagation rate of corrosion, when traversing defects are formed in the coating.